<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    var reg = /^<.+>.+<.+>$/;
        
        var str = "<p class='demo'>hello JavaScript</p>".replace(/^<.+>(.+)<.+>$/,function(match,$1){
            return $1;
        })
        console.log(str);
  </script>
</body>
</html>